Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, generator, gearbox, nacelle, and one or more rotor blades. The rotor blades capture kinetic energy of wind using known airfoil principles. For example, rotor blades typically have the cross-sectional profile of an airfoil such that, during operation, air flows over the blade producing a pressure difference between the sides. Consequently, a lift force, which is directed from a pressure side towards a suction side, acts on the blade. The lift force generates torque on the main rotor shaft, which is geared to a generator for producing electricity. In addition, a plurality of the wind turbines may be arranged in a predetermined geological location and electrically connected together to form a wind farm.
During operation, wind impacts the rotor blades of the wind turbine and the blades transform wind energy into a mechanical rotational torque that rotatably drives a low-speed shaft. The low-speed shaft is configured to drive the gearbox that subsequently steps up the low rotational speed of the low-speed shaft to drive a high-speed shaft at an increased rotational speed. The high-speed shaft is generally rotatably coupled to a generator so as to rotatably drive a generator rotor. As such, a rotating magnetic field may be induced by the generator rotor and a voltage may be induced within a generator stator that is magnetically coupled to the generator rotor. In certain configurations, the associated electrical power can be transmitted to a turbine transformer that is typically connected to a power grid via a grid breaker. Thus, the turbine transformer steps up the voltage amplitude of the electrical power such that the transformed electrical power may be further transmitted to the power grid.
In many wind turbines, the generator rotor may be electrically coupled to a bi-directional power converter that includes a rotor side converter joined to a line side converter via a regulated DC link. More specifically, some wind turbines, such as wind-driven doubly-fed induction generator (DFIG) systems or full power conversion systems, may include a power converter with an AC-DC-AC topology.
The DFIG can be configured to provide both active or real power (measured in Watts) and reactive power (measured in VARs). For example, by controlling the rotor side converter, the real and reactive power generated by the stator can be controlled. Similarly, by controlling the line side converter, the real and reactive power generated by the line side converter can be controlled. Thus, the combined amount of reactive power generated by the DFIG wind turbine system can be controlled to meet a reactive power production requirement, such as a reactive power production requirement set by a dispatch control system of a utility company. In a typical configuration, the stator of a DFIG can be configured to supply the reactive power for the system unless the stator runs out of current margin, in which case, the line side converter can be used to help make reactive power to meet the reactive power production requirement.
In view of the foregoing, it would be advantageous to extend the reactive power capability of a wind farm to meet dynamic requirements.